You are correct in the fact that USDA rural devlopment loans do favor ubc or irc modular homes. It is almost impossible to get a hud code manufactured home refinanced after they have been located on a site more than 2 years.
The newer modular homes have the appearance of site or stick built homes, and often it is difficult to tell them apart. Manufactured home still look like the double wide trailer they were first built as.
I hope this helps answer some of the ?? you may have. I have studied both types of homes as a certified residential appraiser. Jane